1. Field of the Invention
This invention relates to antennas and, more particularly, to a novel and highly effective antenna that strikes an effective compromise between the dielectric constant and loss tangent of a separator provided between a signal radiator and a ground plane, and does so at an exceptionally low coast.
2. Description of the Prior Art
Those skilled in the art of antenna design are aware what there is a tradeoff between the dielectric constant and the loss tangent of a separator provided between the signal radiator and the ground plane of, for example, a patch antenna. A high dielectric constant is desirable because it enables reduction of the physical dimensions of the antenna. A low loss tangent is desirable because it enables an increase in the gain of the antenna. Unfortunately, measures taken to increase the dielectric constant tend to increase the loss tangent, and measures taken to reduce the loss tangent tend to reduce the dielectric constant.
Consider a plane wave propagating in a lossy dielectric. Maxwell's equations for a lossy region are EQU V.times.E=-j.omega..mu.H EQU V.times.H=j.omega..epsilon.E+.sigma.E
where E and H are the electric and magnetic fields, respectively, expressed as vectors; .omega. is the angular frequency; .mu. is the permeability; .epsilon. is the permitivity; and .sigma. is the conductivity. The second equation may be written in the form ##EQU1##
The quantity .epsilon.' is called the relative dielectric constant and the ratio .epsilon."/.epsilon.' is called the loss tangent, denoted tan .delta.. It is called a loss tangent because it is a measure of the ohmic loss in the medium and thus is a measure of the quality of the dielectric.
The dielectric constant affects the dimensions of the distributed circuit components, and the loss tangent affects the loss in the circuit. In the case of the a microstrip patch antenna, a higher dielectric constant allows the patch to be smaller; however, a higher loss tangent reduces the gain of the antenna. While the gain of an antenna is often more important than its size, one would like to obtain a dielectric material that had both a high dielectric constant (for small size) and a low loss tangent (for high gain). In conventional practice, less-than-ideal choices must often be made.
The dielectric constant and loss tangent of some commercial materials employed in a conventional manner are shown in Table 1.
TABLE 1 DIELECTRIC MATERIAL CONSTANT LOSS TANGENT Air 1.00 0.0001 RT/Duroid .RTM. 5880 2.20 0.0009 FR-4 4.20 0.0300
As Table 1 shows, air has a dielectric constant of 1.00 and a loss tangent of 0.0001. A patch antenna employing air as a separator may be taken as a reference to which other designs may be compared.
RT/Duroid.RTM. 5880, which is a registered trademark of Rogers Corporation for a material generically described as PTFE and reinforcing glass fibers, can also be employed as a separator between the signal radiator and the ground plane of a patch antenna. As Table 1 shows, RT/Duroid.RTM. 5880 has a dielectric constant of 2.20 and a loss tangent of 0.0009. While the dielectric constant is good, enabling a reduction in the size of the antenna as compared to an antenna employing air as the dielectric, the loss tangent is undesirably high and compromises the antenna gain. Moreover, RT/Duroid.RTM. 5880 is quite expensive and in many instances not economical for commercial use as a separator in a patch antenna assembly.
FR-4, which is a generic name for an inexpensive glass/epoxy laminate, described as a highly cross linked, brominated epoxy resin reinforced with woven glass cloth, can also be employed as a separator between the signal radiator and the ground plane of a patch antenna. As Table 1 shows, FR-4 has a dielectric constant of 4.20 and a loss tangent of 0.0300. While the dielectric constant is excellent, the loss tangent is high. Despite the low cost of FR-4, its high loss tangent renders it undesirable in conventional use as a separator between the radiating element and the ground plane of a patch antenna.
Many other materials have been tried as dielectric separators, but all have left something to be desired from the standpoint of dielectric constant, loss tangent, cost, weight, physical dimensions, or all of the above.